MTU settings for Vigor 2860

Can anyone tell me the correct MTU settings for a DrayTek Vigor 2860 router (connected direct, no BT modem)?I'm using 1442 at the moment and things are working but is that the highest I can go? I've tried 1458 and 1492 but they won't work.Can someone confirm that 1442 is the correct setting or do I have a problem somewhere?Thanks.